This project investigates biobehavioral development through comparative longitudinal study of rhesus and capuchin monkeys, with special emphasis on characterizing individual differences in behavioral and physiological responses to mild environmental challenges and on determining long-term developmental consequences for individuals reared in different physical and social environments. Analyses completed this past year provided detailed evidence that both genetic and environmental factors differentially influence developmental processes in several behavioral and physiological domains at different stages of ontogeny, in different settings, and in different species. Studies of differentially reared offspring of rhesus monkeys selectively bred for unusually high or low CSF levels of the serotonin metabolite 5-HIAA revealed major blood chemistry and behavioral differences as a function or rearing condition, but not pedigree, in early infancy, whereas in late infancy clear-cut heritable differences became apparent and were predictive of both behavioral and physiological patterns during childhood and adolescence. The relationship between CSF levels of 5-HIAA and measures of social competence, dominance, risk-taking, and aggressive interactions were shown to be similar for adolescent and adult rhesus monkeys living in laboratory settings or residing in the wild. Young rhesus monkey females reared from birth by foster mothers more closely resembled their biological mothers than their their foster mothers in terms of adrenocortical and monoamine responses to challenge; in contrast, their patterns of care of first-born offspring more closely resembled those displayed by their foster mothers. New studies of biobehavioral development in capuchin monkeys revealed ontogenic changes in activity state profiled mirroring those of rhesus monkeys in the nature and developmental course of mother-infant relationships.